[clang-format] Don't require deduction guides to be templates

[rymiel]

Summary

The C++ standard doesn't require that class template deduction guides
be templates themselves, but previously isDeductionGuide would assert
for the existence of a template closer or requires clause closer before
the deduction guide declaration.

This patch simply removes that check. Because of this, a test which
asserted that x()->x<1>; *isn't* a deduction guide failed, as it is
now formatted as a deduction guide. However, as @JohelEGP demonstrated,
it is [possible to make that a viable deduction guide][1].

Thus, that test has been removed, but other tests related to
non-template class template deduction guides have been added.

Fixes https://github.com/llvm/llvm-project/issues/55142

[1]: https://compiler-explorer.com/z/Wx3K6d5K9

[HazardyKnusperkeks]

Seems to be okay.

clang/lib/Format/TokenAnnotator.cpp

if (Current.Previous && Current.Previous->is(tok::r_paren) &&
       if (TemplateCloser && TemplateCloser->Next &&
           TemplateCloser->Next->is(tok::semi) &&
           Current.Previous->MatchingParen) {
         // Determine if the identifier `A` prior to the A<..>; is the same as
         // prior to the A(..)
         FormatToken *LeadingIdentifier =
             Current.Previous->MatchingParen->Previous;
 
-        // Differentiate a deduction guide by seeing the
-        // > of the template prior to the leading identifier.
-        if (LeadingIdentifier) {
-          FormatToken *PriorLeadingIdentifier = LeadingIdentifier->Previous;
-          // Skip back past explicit decoration
-          if (PriorLeadingIdentifier &&
-              PriorLeadingIdentifier->is(tok::kw_explicit)) {
-            PriorLeadingIdentifier = PriorLeadingIdentifier->Previous;
-          }
-
-          return PriorLeadingIdentifier &&
-                 (PriorLeadingIdentifier->is(TT_TemplateCloser) ||
-                  PriorLeadingIdentifier->ClosesRequiresClause) &&
-                 LeadingIdentifier->TokenText == Current.Next->TokenText;
-        }
-      }
-    }
+        return LeadingIdentifier &&
     return false;
   }
 
   void determineTokenType(FormatToken &Current) {
     if (!Current.is(TT_Unknown)) {
       // The token type is already known.
       return;
     }

clang/unittests/Format/FormatTest.cpp

TEST_F(FormatTest, DeductionGuides) {
   verifyFormat("template <class T> A() -> A<decltype(p->foo<3>())>;");
   verifyFormat("template <class T> A() -> A<decltype(foo<traits<1>>)>;");
   verifyFormat("template <class T> A() -> A<sizeof(p->foo<1>)>;");
   verifyFormat("template <class T> A() -> A<(3 < 2)>;");
   verifyFormat("template <class T> A() -> A<((3) < (2))>;");
   verifyFormat("template <class T> x() -> x<1>;");
   verifyFormat("template <class T> explicit x(T &) -> x<1>;");
 
+  verifyFormat("A(const char *) -> A<string &>;");
+  verifyFormat("A() -> A<int>;");
+
   // Ensure not deduction guides.
   verifyFormat("c()->f<int>();");
   verifyFormat("x()->foo<1>;");
   verifyFormat("x = p->foo<3>();");
   verifyFormat("x()->x<1>();");
-  verifyFormat("x()->x<1>;");
 }
 
 TEST_F(FormatTest, BreaksFunctionDeclarationsWithTrailingTokens) {
   // Avoid breaking before trailing 'const' or other trailing annotations, if
   // they are not function-like.
   FormatStyle Style = getGoogleStyleWithColumns(47);
   verifyFormat("void someLongFunction(\n"
                "    int someLoooooooooooooongParameter) const {\n}",